Case Study: Extending the life of gear unit

The bearings, monitoring system or oil system have, of course, been upgraded constantly over the years to remain state-of-the-art. However, the basic principle of the current CPU gears is still based on the original 1966 design. Martin Baechler details the overhaul process of one such two-stage planet gear unit that was recently carried out at Orient Cement.

The first two-stage planet gear unit for the cement industry was developed in 1966 by MAAG ZahnrSder AG (now FLSmidth MAAG Gear) in Zurich, Switzerland. These gear units of type CPU consist of two series-connected planet stages, each with 3 planets and a rotating planet carrier. They replaced traditional, multistage, spur gear units for the central drive of ball mills. Thanks to their higher power density and better efficiency, these gear units are able to transfer more power and take up less space.

This design is very robust and durable. It is proved by the fact that the first two units built in 1967 are still in use in Belgium after over 46 years of operation. More than just periodic inspection and maintenance work is required if a gear unit is to be operated far beyond its normal service life under the tough environmental and operating conditions in the cement industry. The service life can only be extended if the entire gear unit is completely overhauled. Overhauls reduce the overall wear in the gear unit. The wear parts are replaced and the gear unit is serviced so that it is ready for the next operating period. If overhauls are planned in advance, they can be carried out in periods of low demand and also ensure that the gear unit continues to operate without problems. They help reduce maintenance costs, extend the service life of the plant and contribute to avoiding unexpected downtime. Orient Cement in India recently benefited from all of these advantages.

Orient Cement, a far-sighted cement producer

Orient Cement was founded in 1979. It has operated a cement factory in Devapur, in Adilabad district in central India since 1982, and a split-grinding unit in Jalgaon, Maharashtra, since 1997. Orient Cement pursues a clear growth strategy. Between 1990 and 2009, total production capacity was continuously increased to 5 million tonnes of cement a year by upgrading the existing cement lines and adding a new line. To achieve the target of 15 million tonnes a year by 2020, Orient Cements focus is not only on the construction of new production sites. It is equally important to secure the existing production capacity. This requires the maintenance of the existing systems. Only high-quality, well-maintained production plants are able to produce the desired quality and quantity of cement reliably and with optimum efficiency. For this reason, Orient Cement recently had the gear unit of a raw meal mill completely overhauled.

The gear unit, of type CPU-30-2, was designed and produced in 1987 by FLSmidth MAAG Gear in Switzerland. The design differs from the standard CPU series because at that time motors with the required output of 3800 kW were not available. A drive with two parallel motors, each with an output of 1900 kW, was developed for Orient Cement. As its first reduction, this CPU does not have a planet stage. It has two input pinions that mesh with a common gearwheel. The accumulated power is transferred to the second stage, a planetary gear with rotating planet carrier, via a gear coupling. The input speed of 992 RPM is reduced to the mill speed of 14.76 RPM in the two gear stages.

Complete overhaul of a two-stage central drive

After first operation, the gear unit was inspected during the warranty period after 6,000 and 12,000 operating hours. It then ran for more than 10 years without problems, maintained by the local maintenance team. The first major inspection by FLSmidth MAAG Gear took place in 2005 and the gear unit was found to be in good condition.

As recommended by FLSmidth MAAG Gear, a complete overhaul was carried out in September 2013 after approximately 134,000 operating hours or a service life of 26 years. Roughly six months beforehand, a service engineer from FLSmidth MAAG Gear produced an initial analysis of the gear unit on site and established that all necessary spare parts for the overhaul were available locally.

When the service engineers arrived in Devapur, the gear unit had already been prepared for the overhaul. The raw mill was stopped and the auxiliary drives and the power supply of the main motors had been removed by Orient Cement employees. The overhaul work was completed in just 10 days. The oil pipes and connection couplings had to be removed first to allow the gearing and the bearings of the two gear stages to be inspected. The three planet axles in the second stage also serve as journals and are therefore coated with a layer of white metal. As part of this overhaul, Orient Cement inspected the journals using ultrasound to ensure that the white metal layer was still in perfect condition. The gearing of the sun pinion of the second stage was also inspected using ultrasound as it is the gearing subject to the highest load in the gear. All sliding bearings in both gear stages were completely replaced. The gear unit was also cleaned and small parts and wearing parts were replaced. When the gear stages were assembled, the internal alignment of the gearwheels and the tooth contacts were reset.

The gear unit was completely assembled in the last 3 days of the mills downtime. The service engineers first assembled the two gear stages again and set the tooth contact and backlash correctly. The gear unit was provisionally aligned and braced on the foundation. Wearing parts such as O-rings and coupling bolts in the output coupling were replaced before the coupling was installed. The cleaned and inspected couplings were filled with new lubricating grease and the alignment between the motors, the gear unit and the mill was restored. The oil pipes were then connected and flushed with the oil. The rotational direction test on the motors was the last job before the load test and the resumption of operation of the raw meal mill.

Conclusion

After this 10-day complete overhaul, the Orient Cement gear unit has new bearings, all the wearing parts have been replaced and the alignment of the gearing has been restored to the original setpoint. The alignment with the drive motors and the mill has also been checked and is in the permissible range. The gear unit is not new but it is fit for the next stage of its life. With good care and the necessary maintenance, it will continue to operate well until the next complete overhaul is due after 80,000 hrs or 12 years.